This invention relates to the art of amplifiers and, more particularly, to improvements to polyphase pulse duration modulation (PDM) amplifiers to achieve distortion reduction.
Although the invention will be described herein with particular reference to a modulator circuit for an RF transmitter, it is to be appreciated that the invention has broader applications and may be used in conjunction with signal amplification in other areas such as high fidelity audio systems and public address systems, etc.
In order to achieve efficiency of operation in power amplifiers, pulse duration modulation (PDM) is frequently employed. In such amplifiers an input signal, such as an audio signal, is used to pulse modulate a carrier signal, and the resulting PDM signal is amplified. The amplified PDM signal is then filtered to recover a demodulated signal corresponding to an amplified version of the input audio signal.
A polyphase PDM amplifier is disclosed in U.S. Pat. No. 4,164,714 to H. Swanson and assigned to the assignee herein. That patent discloses a system wherein an amplified signal is formed by combining the outputs of plural parallel-connected PDM amplifiers. All of the amplifiers respond to a common input signal, but have differently phased carrier signals. Polyphase PDM amplifiers of this type enjoy various advantages over earlier, single phase PDM amplifiers. For example, amplifier elements employed in polyphase PDM amplifiers operate at relatively low switching frequencies and power levels, permitting semiconductor devices to be used as the active amplifier elements instead of vacuum tube devices.
The switching frequencies at which the PDM amplifiers operate serve to drive a switching device such as a semiconductor so as to generate a train of square-wave pulses of fixed magnitude but of variable pulse width in dependence on the on-off time durations of the semiconductor switching device. This is followed by a filter stage used in recovering the amplified audio signal from the pulse duration modulated signal. The Swanson patent, supra, employs a semiconductor switch and a series connected filtering stage for each PDM amplifier. These are connected in parallel and serve as a combiner in recovering the amplified audio signal during which the frequency component corresponding to the switching signal is removed without significantly effecting the frequency spectrum of the audio component. This combined signal is then supplied to a load in the form of an RF modulator for modulating a carrier signal from an oscillator for transmission purposes. It has been found, however, that the effective capacitance of each semiconductor switching device will cause distortion in the square-wave signal in each stage if the width of the driving pulse is narrow. Thus, in such a system, the current passing through the filter stage will be very small during this time resulting in a much slower charging of the effective capacitance than when the width of the PDM pulse is much wider (longer on time).